Optical coherence (2011-)

The closest approximation of the ideal 'optical ray' in the lab are paraxial optical beams, such as the Gaussian beam. Such a beam contains many plane wave components which have a certain spread of wave vectors around the central angle (direction). It is the coherent superposition of these plane waves which makes low-divergence beams possible but also leads to surprising effects upon reflection: the reflected beam is displaced with respect to the ray-optics prediction.

We can also reduce the degree of spatial coherence of an optical beam, which must then be described as a stochastic mixture state of many optical beams with the same statistical properties. These beams are called 'Gaussian Schell-model' beams, because they still behave largely like an ordinary beam. If we look at one realisation (snapshot) of such a beam, we find optical speckle appearing if the degree of coherence is reduced:

It had been speculated since many years if the orbital angular momentum of light might be usable in astronomy. The promise is big: Possibly, one could retrieve information about the spin of Kerr black holes, or other possibly undiscovered space-twisting objects. But, the observation of OAM requires a suitable light source with sufficient spatial coherence so that on earth a nonzero OAM is detectable. So, it's about the propagation of spatial coherence with a “light-twisting object” in between. Surprisingly, this has not been studied before and there are no simple equations describing this. We did a lab-scale experiment and were able to find intuitive equations explaining the experimental observations.

What is our result, is OAM usable in space? As experts might have expected, it is a great challenge because a detailed balance of the number of modes must be met: The light twisting object should be illuminated with as few as possible modes (i.e., with light with a large coherence length), while it is required to receive from the very same object as many modes as possible (i.e., we must resolve it in astronomical imaging). However, we found several subtleties; for very specific applications, I predict that OAM might turnout to be useful indeed.

The Role of Spatial Coherence and Orbital Angular Momentum of Light in Astronomy, Phys. Rev. A 90, 063801 (2014), Arxiv:1404.6404 (2014).

The influence of spatial coherence on optical phenomena is always very important for applications since one can possibly replace a laser (coherent) by a LED (incoherent)! For beam shifts, it was also experimentally not tested if coherence plays a role. We found that spatial beam shifts, which have successfully been tested in bio sensing, are not affected by the reduced spatial coherence. More information:

Spatial Coherence and Optical Beam Shifts, Phys. Rev. Lett. 109, 213901 (2012) (on the cover!), preprint: Arxiv:1207.4364 (2012)

A beam with reduced spatial coherence also enables a curious experiment, it firstly allows us to see optical beam shift phenomena with the naked eye at a single standard air-glass interface! Below, two images are shown of the reflected field of a beam with reduced spatial coherence; for the two orthogonal linear polarizations (beam shifts depend strongly on polarization). Because of the granularity introduced by the speckle pattern, you can see that speckle on the left edge are more intense in the right picture than in the left. Therefore, the beam is shifted to the 'right' in the left picture.